1. Field of the Invention
The present invention relates to an optical recording apparatus for performing recording by use of multi-pulse trains, that is, pulse train recording onto a recording medium, and a method for controlling laser power therein. Specifically, the present invention relates to an optical recording apparatus and the like in which light returned from the recording medium is detected and an average level signal of the detected signal thereof is obtained, and then, laser power is controlled in such a manner that the level of the average level signal becomes a predetermined level thereby enabling to stably control the write power during a recording operation, even in the case of performing pulse train recording.
2. Description of the Related Art
Conventionally, a CD-Recordable (CD-R) drive as an optical recording apparatus employs write strategies referred to as row and column write strategies (i.e. write power controlling methods), in order to form pits in correct lengths which correspond to pulses of recording data RD onto a disc (i.e. CD-R).
Description will be made on the case where recording is performed by employing a row write strategy, with reference to FIGS. 1A to 1E. FIG. 1A shows a basic channel clock CLK with a period T. FIG. 1B shows recording data RD. The recording data RD has been obtained by performing Non-Return to Zero Inverted (NRZI) conversion for a modulation signal resulted from Eight to fourteen Modulation (EFM). As conventionally known, in the recording data RD, each period with pulses “1” and “0” has a time length from 3T to 11T, respectively.
In the row write strategy, as shown in FIG. 1C, a portion “1” corresponding to only 1T is deleted from the time lengths 3T to 11T of the recording data RD. Then, a time corresponding to a row time is added only to the portion with a time length 3T to render the laser power to the write power, and in this state, pits are formed on the disc, as shown in FIG. 1E. FIG. 1D shows a write RF signal WRF obtained by detecting the light returned from the disc in a recording operation.
Next, description will be made on the case where recording is performed by employing the column write strategy, with reference to FIGS. 2A to 2E. FIG. 2A shows a basic channel clock CLK with a period T. FIG. 2B shows recording data RD. As has been described above, the recording data RD has been obtained by performing Non-Return to Zero Inverted conversion for a modulation signal resulted from Eight to fourteen Modulation (EUM), and each period with pulses “1” and “0” has a time length from 3T to 11T, respectively.
In the column write strategy, as shown in FIG. 2C, a portion “1” corresponding to only 1T is deleted from the time lengths 3T to 11T of the recording data RD. Then, only the first column time is provided with a peak power larger than the write power by a level corresponding to the column amount, while the remaining column time is provided with a write power, and in this state, pits are formed on the disc, as shown in FIG. 2E. FIG. 2D shows a write RF signal WRF obtained by detecting the light returned from the disc in a recording operation.
The reason why a portion “1” corresponding to only 1T is deleted from the time lengths 3T to 11IT of the recording data RD when recording is performed by employing the row write strategy or the column write strategy described above is as follows. Since the spot of the laser beam formed on the disc has a certain size, the pits actually formed on the disc respectively have a length resulted from adding an extended amount of the pits caused by the shape of the spot of the laser beam to the pit length corresponding to the irradiation time of the laser beam at a write power.
Description will be made on the waveform of the write RF signal WRF, with reference to FIGS. 3A and 3B. FIG. 3B shows a laser power, and FIG. 3A shows a write RF signal WRF obtained corresponding to the laser power.
A Read level is a level of the RF signal in the read state before writing is performed, and is determined by a read power and a reflection ratio (i.e. sensitivity) of the disc (i.e. medium). A peak level is a level of the RF signal immediately after the laser power is switched to the write power. Since a CUR is designed for thermal recording, pits are not formed immediately after the laser power is switched to the write power. During the short period until the pits are formed, the level of the RF signal increases to the level determined by the write power and the reflection ratio of the disc.
The pit level is a level at which the level of the RF signal converges when it starts to gradually decrease from the peak level after the laser power is switched to the write power. The reason why the level of the RF signal gradually decreases is that the reflection ratio of the disc decreases due to the formation of pits. The bottom level is a level of the RF signal immediately after the laser power is switched to the read power. The level of the RF signal decreases to the bottom level, and after that, passes through the read setting time and increases to the read level.
FIGS. 4A to 4C respectively show a relationship between a thermal time constant of a disc (i.e. medium) τ and a waveform of the write RF signal WRF.
FIG. 4A shows the case where the relationship of τ=0 is established. In this case, the relationship of heating amount=radiation amount is established, and there is no movement of the pit forming point. Therefore, there is no decrease in the pit level. In this state, it is possible to form pits whose lengths are proportional to the pulse time, independent of the length of write pulse, and jitter is unlikely to occur.
FIG. 4B shows the case where the relationship of τ>0 is established. In this case, the relationship of heating amount>radiation amount is established, and there is a slight movement of the pit forming point. Therefore, there is a slight decrease in the pit level. In this state, the pit length gradually becomes disproportional to the pulse length of the write pulse, and jitter occurs.
FIG. 4C shows the case where the relationship of τ>>0 is established. In this case, the relationship of heating amount>>radiation amount is established, and there is a large movement of the pit forming point. Therefore, there is a sharp decrease in the pit level. In this state, large jitter occurs.
When recording is performed by employing the row write strategy described above, as shown in FIG. 1D, the pit level gradually decreases. Therefore, the pit forming point is deviated, resulting in jitter occurrence. Contrary to this, when recording is performed by employing the column write strategy, as has been described above, the signal level is increased to the peak power first, and then, is decreased to the write power to establish the relationship of heating amount=radiation amount. In this manner, the deviation of the pit forming point can be prevented. Therefore, all the pits from 3T to 11T can be written at (n−1)T, thereby avoiding the occurrence of jitter.
In addition, in order to form a recording mark in a correct length which corresponds to the pulse of the recording data RD, a pulse train write strategy has been conventionally employed in a CD-ReWritable (CD-RW) drive, although not employed in a CD-R drive. The pulse train write strategy performs recording by converting the recording data RD into multi pulse trains. As compared with the cases of single pulse recording (i.e. row write strategy and column write strategy), the pulse train write strategy can lower the shifts in both the front and rear edges of the recording mark by closely controlling the on-off operation of the laser to suppress the influence of thermal storage to a minimum value.
Description will be made on the case where recording is performed by employing the pulse train write strategy, with reference to FIGS. 5A to 5E. FIG. 5A shows a basic channel clock CLK with a period T. FIG. 5B shows recording data RD. As has been described above, the recording data RD has been obtained by performing Non-Return to Zero Inverted conversion for a modulation signal resulted from Eight to fourteen Modulation (EFM), and each period with pulses “1” and “0” has a time length from 3T to 11T, respectively.
In the pulse train write strategy, as shown in FIG. 5C, the portion of “1” having a length from 3T to 11T of the recording data RD is converted into multi-pulse trains to render the laser power to the write power intermittently, and then, a recording mark is formed on the disc as shown in FIG. 5E. FIG. 5D shows a write RF signal WRF obtained by detecting the light returned from the disc at the time of recording.
In the CD-R drive described above, the laser power is controlled, that is, running optimum power control (R-OPC) is performed during recording operation. The purpose of the R-OPC is to absorb the variations in reflection ratios at inner and outer peripheries (non-uniformity in coating of a recording layer and the like) of the disc (CD-R), a change in the distribution of spot strengths at coma aberration generated as a result of the skew of the disc, a change in the laser wavelength resulted from an increase in temperature, and the like.
In the CD-R drive, before writing is s trial writing is usually performed in a power caribration area (PCA) provided to the disc to obtain an optimum write power. However, due to the reason described above, if the writing is performed at the same write power from the initiation to the end of the writing, it is impossible to perform writing in such a manner that the jitter is always suppressed to a minimum value.
Therefore, the optimum power is obtained in the PCA (i.e. the value at which the asymmetry becomes optimum), and simultaneously, the pit level at this time is obtained as an optimum pit level. Then the write power is controlled in such a manner that the pit level becomes optimum during recording operation. This arrangement enables recording to be always performed in the same writing manner.
In order to control the write power in such a manner that the pit level becomes optimum during recording operation, it is necessary to detect the pit level during recording operation The pit level is detected by sampling the write RF signal WRF obtained in the portion with the predetermined pulse width of the recording data RD at the sample timing corresponding to the pulse width.
FIG. 6A shows a write RF signal WRF corresponding to the portions with pulse widths 4T, 11T in the case where recording is performed by employing the column write strategy. FIG. 6B shows a sample pulse for detecting a pit level at the portion with each of the pulse widths, and the sampling is performed at the timing when the writing RF signal WRF becomes at a constant level. The write RF signal WRF becomes at a constant level at the portion with a relatively wide pulse width, and actually, the sampling of pit level is performed corresponding to the portion with the pulse width of 6T or larger for example.
When recording is performed by employing the column write strategy as described above, R-OPC can be performed satisfactorily by detecting the pit level of the write RF signal WFR which corresponds to the portion with a predetermined pulse width.
However, when recording is performed by employing the pulse train write strategy, it is difficult to detect the pit level of the write RF signal WRF which corresponds to the portion with a predetermined pulse width unlike the above described case. As a result, it is impossible to perform R-OPC.
Specifically, when the recording is performed by employing the pulse train write strategy, as shown in FIG. 7A, the write RF signal WRF which corresponds to the portions with the pulse widths 4T, 11T varies corresponding to the write pulse (i.e. multi-pulse trains). Therefore, if the pit level of the portion with each of the pulse widths is detected by the sample pulse shown in FIG. 7B, it is impossible to obtain a detected pit level in a stable manner, and it is impossible to perform R-OPC.